Heat-shrinkable films have a variety of uses. For example, they are often used to wrap groups of bottles, cans or cartons. Typically, films are applied at room temperature and then placed under a heat source to shrink. The shrink properties of the film including shrinkage in the transverse direction are important for these applications. Films of low density polyethylene (LDPE) are commonly used because of the excellent heat-shrink and optical properties of LDPE. However, LDPE has relatively poor strength and low modulus. To improve costs, there is an emphasis on using thinner films. As films become thinner, the detriments of LDPE become more significant. There have been different approaches to solve this problem.
One approach has been to make the film from a polymer blend. For example, U.S. Pat. No. 6,306,969 describes a film from a blend of polyethylene components that are closely matched in density and exhibit shrink in the direction of orientation. U.S. Pat. No. 7,439,306 teaches a method based upon Vicat softening points to select components to make a polyethylene blend useful for shrink films. While making a film from a blend can improve some properties, other properties are diminished.
Another approach has been to prepare multilayer films. For example, U.S. Pat. No. 6,368,545 teaches that core layers may be used to provide strength, impact resistance, and other properties to a film and teaches that haze can be lowered by extruding the core layer at a temperature at least 30° C. higher than the skin layers. There is no discussion of shrink. U.S. Pat. No. 6,187,397 teaches heat-shrinkable polyethylene film. In the background discussion, they teach that films have been obtained by coextrusion of three layers, more specifically a central layer of LDPE sandwiched between two linear low density polyethylene layers. They state that the central layer may include HDPE to impart stiffness, but generally the central layer comprises more than 50% by weight of LDPE. U.S. Pat. Appl. Publ. No. 2005/0064218 discloses a multilayer film containing metallocene-rich skin layers and a core containing 1-50% by weight HDPE. They state that the HDPE may be prepared by any process and Ziegler-Natta prepared HDPE is preferred. No shrink values are given. U.S. Pat. No. 7,442,786 teaches a film structure containing at least two layers, a metallocene polyethylene layer and a layer comprising from 99 to 50% by weight LDPE and 1 to 50% by weight HDPE. They state that the HDPE may be prepared by any process and Ziegler-Natta prepared HDPE is preferred. No shrink values are given.
LDPE films with good shrink properties are known, and it is known to make multilayer films and that HDPE can be used to improve the stiffness. Apparently, heat-shrinkable films with good stiffness that have good shrink properties in the transverse direction are not known.